These studies are aimed at understanding, (1) the mechanism of transport of protein-bound steroid and thyroid hormones through the brain capillary wall, i.e., the blood-brain barrier (BBB), and (2) the action of circulating hormones on brain in special regard to the hormonal medulation of BBB nutrient transport. Since many pathways of brain metabolism are influenced by precursor availability, the adaptations caused by hormones in regard to BBB nutrient transport may have important influences on brain function.